I have been trying to make sure that my daughter hears me say with some frequency "I don't know (but I'm curious)" and "I'm wrong (but I'd like to know what's right)," since that's the best way for her to learn to say these things when appropriate. Looking in the big picture well beyond math, the world would be a better place if people would say those things more frequently. But math is one of the best places to come to terms with these sentiments (which is one of the many reasons some people dislike it).
Children are sometimes OK with not knowing, but hate being wrong. I don't want my daughter to have the impression that I know everything, so I'm really happy when she asks science questions I don't have good answers for. That's not going to happen in math for a while, but I will sometimes intentionally lead her astray and push along until she catches me - that has a big element of fun to it (she gives me these looks when she starts to suspect me) and catching me being wrong is a step, I think, in her developing the ability to catch her own mistakes.
